Fuel cells are especially envisaged as an energy source for future mass-produced motor vehicles. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. A fuel cell comprises a stack of several cells in series. Each cell generates a voltage of the order of 1 Volt and their stacking enables the generation of a power supply voltage of a higher level, for example of the order of 100 volts.
Among the known types of fuel cells, we can cite especially the proton-exchange membrane, or “PEM.” Such fuel cells have particularly interesting properties of compactness. Each cell has an electrolytic membrane enabling only the passage of protons and not the passage of electrons. The membrane enables the separation of the cell into two compartments to prevent direct reaction between the reactant gases. The membrane comprises an anode on a first face and a cathode on a second face, this assembly being usually designated by the term “membrane/electrode assembly” or “MEA.”
Within the fuel cell's active area, at the anode, molecular hydrogen or hydrogen (H2) used as fuel is ionized to produce protons passing through the membrane. The electrons produced by this reaction migrate to a flow plate and then pass through an electrical circuit external to the cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water.
The cell can comprise several flow plates, for example made of metal, stacked on one another. The membrane is positioned between two flow plates. The flow plates can comprise channels and holes to guide the reactants and products to and from the membrane. The plates are also electrically conductive so as to form collectors for the electrons generated at the anode. Gas diffusion layers are interposed between the electrodes and the flow plates and are in contact with the flow plates.
The flow plates are in contact with highly acidic solutions. On the cathode side, the plate is subjected to air under pressure in a highly oxidizing environment. On the anode side, the plate is contact with hydrogen. In such conditions, the metal plates undergo corrosion phenomena. The corrosion of a plate causes, firstly, the emission of metal ions that impair the working of the electrolytic membrane. The corrosion of the plate, secondly, gives rise to the formation of an insulating oxide layer on the metal, thus increasing its contact resistance relative to the gas diffusion layer. The electrical resistance between the flow plate and the gas diffusion layer is then increased. These phenomena cause a reduction of performance of the fuel cell. The flow plates must therefore have a high electrical conductivity while at the same time avoiding phenomena of oxidation and corrosion.
The industrial-scale development of fuel cells implies a great increase in the costs of manufacture of the different components. In particular, the cost of the flow plates is as yet unacceptable for large-scale use.
To reduce their cost, the flow plates are generally formed as bipolar plates including two flow plates. In one industrially tested solution, two metal sheets made of stainless steel are pressed and joined back-to-back by laser welding to form flow plates for adjacent cells. The welds are made at the bottom of channels, and the bottoms of the channels of the two back-to-back metal sheets are placed in contact. In order to reduce manufacturing costs, the back-to-back metal sheets have the same geometry.
The document US2006046130 describes a fuel cell intended to limit the influence of the variation of compressive forces on a stack of cells. The bipolar plates are formed by assembling two metal sheets. Each metal sheet has a relief to form gas flow channels. A multitude of adjacent channels extends along a same direction. Two metal sheets are joined together in placing the bottom of certain channels of these sheets in contact and then making welds in these bottoms. To enable the absorption of the variations in compressive forces, spaces are made between the bottoms of certain channels of two joined metal sheets. To homogenize the absorption of the variations of compression along the direction transversal to the channels, the spaces of the successive bipolar plates are offset.
In practice, prior-art fuel cells have relative heterogeneous functioning at the level of the membrane/electrode assembly. This heterogeneity can be explained especially by the development of the humidity of the gases between the input and the output of the membrane/electrode assembly. This heterogeneity induces a local increase in the current density, fostering localized corrosion of carbon.